Electrophoresis refers to the movement of charged particles suspended in a suspending fluid under the influence of an applied electric field. If the electric field is applied between electrodes in a cell, the particles will migrate, depending on their polarity, to either the cathode or the anode while the suspending fluid remains essentially stationary. When a voltage is applied to selected electrodes, the particles in the suspending fluid migrate under the influence of the electric field to the electrode having a polarity opposite from their own. Charging of electrodes can be used to create an image or pattern from the electrophoretic particles.
In some electrophoretic inks, the particles and suspending fluid can be contained within capsules or microcapsules. In these kinds of displays, the particles migrate through the suspending fluid within the capsule under the influence of an applied electric field. The capsules can be incorporated within a substance like a binder and applied to a wide variety of substrates, including flexible and non-planar substrates. In an encapsulated electrophoretic display, the binder material surrounds the capsules and may be used to separate the two electrodes defining the display. Examples of such inks are disclosed in U.S. Pat. No. 6,822,782 to Honeyman et al.; U.S. Pat. No. 6,693,621 to Hayakawa et al.; U.S. Pat. No. 6,750,844 to Nakanishi; and U.S. Pat. No. 6,529,313 to Lin et al., the disclosures of which are each totally incorporated herein by reference.
Encapsulated electrophoretic inks may be used in digital document media. Electrophoretic display devices may include a suspension of colored or otherwise visible charged pigment particles dispersed in a dyed insulating medium of contrasting color which is positioned in a cell consisting of two separated parallel and transparent conducting electrodes or plates. In some electrophoretic displays, the capsule has only a single type of visible electrophoretic particle suspended in a contrasting suspending medium. Capsules may be placed between a pair of separated parallel electrodes, at least one of which is transparent, to form a display. One example of an encapsulated electrophoretic ink includes capsules that contain white pigment electrophoretic particles suspended in a dyed or otherwise visible dielectric solvent or other fluid. During use in a display device, the white charged particles in the capsule are transported and packed against one electrode under the influence of an electric field, so that the viewer may see the white reflection from the pigment. When the polarity of the electric field is reversed, the pigment particles are transported away from the transparent electrode and may be packed on the opposite electrode. If the optical density of the dyed fluid is high enough to absorb the light scattered by the particles residing on or near the rear electrode, the observer will perceive the color of the dyed fluid.
During use in a display, packing of the ink or electrophoretic particles in the capsule leads to trapping of the dyed dielectric fluid in the interstices between the electrophoretic particles when they are attracted to the electrode. This dye trapping reduces the image or reflection produced by the pigmented electrophoretic particles. Further, the stability of the suspension of electrophoretic particles in the capsule impacts display quality and device speed. Agglomeration or settling of electrophoretic particles in capsules or display cavities can lead to degradation of the display over time.
There is a need for greater reflectivity in one particle electrophoretic displays to enhance the contrast and readability of images formed using them. There is a need to reduce dye trapping and the agglomeration of electrophoretic particles in displays utilizing single particle encapsulated electrophoretic particles.